A synchronous motor generally has a stator winding energized with alternating current (AC) to produce a rotating magnetic field. The motor also generally has a rotor field winding energized with direct current (DC) excitation from an excitation source to produce a unidirectional magnetic field which interacts with the rotating magnetic field to cause the rotor to rotate in synchronism with the AC frequency.
When the synchronous motor is started, the stator winding acts as a primary winding of a transformer and the field winding acts as a secondary winding of the transformer. As a result, high voltages may be induced in the field winding. These high voltages may cause damage to the field winding and associated components.
In order to eliminate the risk of damage, the field winding is shunted through a field discharge resistor during starting. Just before or after synchronization, the field discharge resistor is disconnected from the field winding to avoid current drain from the excitation source through the field discharge resistor.
Such prior art synchronous motors have further generally included a control system for controlling the application of excitation to the motor. The control system includes a field discharge resistance circuit including the field discharge resistor for discharging induced currents in the field winding during the start up period and a DC excitation circuit for energizing the motor field winding at excitation speed in order to develop the torque required to synchronize the motor. Along with the field discharge resistor, the field discharge resistance circuit includes one or more switching devices which selectively couple the field discharge resistor across the field winding. The DC excitation circuit includes the excitation source (or exciter) and one or more switching devices which selectively couple an exciter to the motor field winding. These switching devices all operate in response to control signals.
The prior art control system further includes a detection circuit for determining when the rotor speed is sufficiently close to the speed of the rotating magnetic field for synchronous lock-in to occur. The detection circuit typically detects the slip signal induced in the field discharge resistor. The rotating magnetic field in the stator induces an alternating current in the field winding which is shorted through the field discharge resistor. The induced field current or slip signal initially has a frequency corresponding to the frequency of the alternating current supplied to the stator winding. The frequency decreases as the motor approaches synchronous speed. The detection circuit operates by detecting the AC frequency of the induced rotor field current in the field discharge resistor.
The prior art control system further includes a field application circuit for controlling the application of excitation to the field winding. DC excitation is applied to the field winding as the rotor approaches synchronous speed to pull the rotor into synchronism with the rotating stator magnetic field with minimum rotor slip. Typically, the DC excitation is applied when motor speed is 90-95% of synchronous speed.
When the frequency of the slip signal detected by the detecting circuit falls below a predetermined threshold known as the lock-in-frequency, the detection circuit provides an indication to the field application circuit. In response, the field application circuit provides control signals to the switching devices to decouple the field discharge resistance from the field winding and to couple the exciter to the field winding.
Prior art control systems, including detection circuits, have included analog circuitry for sensing the slip signal, determining the frequency of the slip signal, determining when to apply excitation to the field winding and for providing control signals in the switching devices. Analog circuitry may readily interface with the very large voltages and currents associated with the synchronous motor.
However, the analog circuitry includes devices such as resistors, capacitors and unijunction transistors and has several limitations. Devices such as resistors and capacitors are manufactured to have a specified resistance or capacitance value plus or minus a specified tolerance. Typical tolerances are 5% or 10% or greater. Where several resistors or capacitors are combined to form a circuit which charges or discharges in response to the slip signal or another control signal, the tolerances of these devices combine. This can make it difficult to design a precision circuit capable of precisely detecting the frequency of the slip signal and in response provide the necessary switching signals. Moreover, devices such as resistors and capacitors have temperature sensitivities. Their respective values vary with temperature. Since the operating conditions of the control system cannot be known beforehand, a control system using analog circuitry must be carefully designed to compensate for temperature variations of the devices which form the circuit.
A further disadvantage of prior art analog control systems for synchronous motors is the inability to precisely control the lock-in frequency. Prior art circuits used analog devices such as a trim pot (a variable resistor) or a trim capacitor (a variable capacitor) to vary the frequency of the detected slip signal at which the field discharge resistance is removed from the circuit and excitation is applied. Trim pots and trim capacitors have tolerance and temperature variation problems similar to those described above. Moreover, since the trim pot and trim capacitors are continuously variable, it is difficult to know with precision what pull-in frequency is being set using these devices.
Accordingly, there is a need in the art for an improved apparatus for applying field excitation to a synchronous electric motor. Further, there is a need in the art for a control system for a synchronous motor which overcomes the noted limitations of prior art control systems.